Aliphatic diisocyanates are major building blocks for the value-added polyurethane products most commonly used in the coatings industry. There are important differences between aromatic and aliphatic diisocyanate monomers. Aromatic diisocyanates, for instance, are much more reactive than their aliphatic diisocyanate counterparts. Urethane products made from aromatic diisocyanate monomers oxidize more easily than those prepared from aliphatic diisocyanates, especially when exposed to UV-light. The higher resistance to UV light-induced degradation of products prepared from aliphatic diisocyanates make them more useful in high quality exterior coatings where gloss and color retention are most important.
Ongoing demand for aliphatic diisocyanates has increased a long felt need to develop a method for making aliphatic diisocyanates that meets the following criteria. First, the method should be highly selective to aliphatic diisocyanate monomers and dimers (uretdiones) such that only an insignificant amount of the aliphatic diisocyanate is wasted. This would result in a high yield of the aliphatic diisocyanate as well as provide a product stream of high purity. Second, the method should operate at a temperature that is lower than the operating temperatures of distillation processes, so that energy costs can be reduced. This would also help to avoid thermal degradation of the product and undesirable side reactions.
Applicants are not aware of any known method that meets this criteria. Hexamethylene diisocyanate, for instance, is ordinarily produced on an industrial scale by phosgenation of 1,6-hexamethylene diamine in the presence of an inert solvent such as chlorobenzene or ortho-dicloro benzene (see. Ullmans Encyklopadie der technischen Chemi, 4.sup.th edition (1977), Volume 14, page 350, et. seq.). After phosgenation, the resulting product is generally subjected to vacuum distillation from which two products are separated: (i) a purified hexamethylene diisocyanate (HDI) product and (ii) a "waste stream" (a stream of unpurified oligomeric products). The purified product is collected and the waste stream, (which often contains an appreciable amount of valuable materials, e.g., aliphatic diisocyanate monomers, uretdiones), is disposed of.
The disposal of valuable aliphatic diisocyanate monomers and uretdiones with the waste stream has long been regarded as a significant shortcoming of known aliphatic diisocyanate production methods. Efforts to recover aliphatic diisocyanate monomers and uretdiones from waste streams by subjecting a waste stream to multiple distillation steps have not been successful. This is because multiple distillation techniques increase the amount of high molecular weight oligomers. Purification efforts also require a considerable expenditure of energy and outlay in apparatus. Further, the distillation procedures produce an undistillable residue that is very expensive to dispose of because it contains aliphatic diisocyanate components, i.e., monomers, uretdiones, and isocyanurates.
U.S. Pat. No. 4,918,220 discloses a method for separating and recovering toluene diisocyanate, an aromatic diisocyanate, from residues formed during the production of toluene diisocyanate with supercritical extraction techniques. The patent is directed exclusively to the recovery and separation of toluene diisocyanate from residues. There is no discussion about how aliphatic diisocyanates can be extracted from residues formed during the production of aliphatic diisocyanates. There is no discussion about the intermolecular interactions of aliphatic diisocyanate waste streams in supercritical fluids.
U.S. Pat. No. 4,871,460 discloses the separation and purification of isocyanate condensates, reaction products of isocyanates that are used to make foams. The patent focuses in applying supercritical extraction techniques to crude reaction mixtures and does not discuss how supercritical extraction techniques can be applied to waste streams. The patent does not discuss how supercritical extraction techniques can be applied to selectively extract aliphatic monomers or uretdiones from waste streams. U.S. Pat. No. 4,871,828 is also directed to the separation and purification of isocyanate condensates.
U.S. Pat. No. 4,864,025 discloses substantially pure isocyanurate/polyisocyanates that are produced by extracting impure cyclotrimerized diisocyanates with an inert gas, either in the liquid or supercritical state. The method involves cyclotrimerizing at least one aliphatic, alicyclic or arylaliphatic diisocyanate (in which the isocyanate groups are not directly linked to an aromatic ring) and removing the excess diisocyanate monomer and dimer formed with an inert gas in the liquid state or supercritical state. The patent is directed primarily to applying supercritical extraction techniques for purifying isocyanurates from crude reaction mixtures. The patent, however, does not teach how supercritical extraction techniques can be applied to selectively extract an appreciable amount, e.g., 85% or more of aliphatic diisocyanate monomers or uretdiones from waste streams. The patent does not discuss the conditions that are necessary to selectively extract aliphatic diisocyanate monomers and uretdiones in a commercial-scale method.
It is an object of the invention to develop a method for making an aliphatic diisocyanate that overcomes the disadvantages of known methods.